This disclosure relates to optical oscillators. Optical parametric oscillators are an important optical source. In general, in the material with second or third nonlinear nonlinearity, the energy of pump light would be transferred to newly generated light with new frequency. The traditional optical parametric oscillator is based on forward (co-propagating) parametric process, such as forward difference frequency generation (DFG) and forward sum frequency generation (SFG) in material of second-order optical nonlinearity, and forward four-wave mixing (FWM) in material of the third-order optical nonlinearity. An optical parametric oscillator using forward parametric process requires cavity structure, such as mirror, Bragg grating and ring resonator, to provide resonance for oscillation. Mirrorless oscillation using backward DFG in a crystal with second-order nonlinearity has been demonstrated (see C. Canalias and V. Pasiskevicius, Nature Photonics 459-462 (2007)).
One prominent optical nonlinearity is FWM in nonlinear material of third-order nonlinearity, which has been investigated when the four optical waves co-propagate, Most FWM optical parametric oscillators are based on forward nonlinear parametric process in nonlinear crystal and waveguide. Mirrorless non-degenerate mirrorless optical parametric oscillator using backward (or counter-propagating) FWM process has been only demonstrated in atom system as nonlinear material.